Conventionally, an emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescence element and an organic electroluminescence element (hereinafter, referred to as an organic EL element). An inorganic electroluminescence element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescence element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence and phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescence element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
As an investigation of an organic EL element for practical use, an organic EL element using phosphorescent emission from an excited triplet state was reported by M. A. Baldo at al in Princeton University, Nature, vol. 395, pp 151-154 (1998). Since then, there was investigated extensively materials which exhibit phosphorescent emission at room temperature and they were reported in Nature, vol. 403, no. 17, pp 750-753 (2000) and the U.S. Pat. No. 6,097,147.
Furthermore, in the organic EL element using phosphorescence luminescence which was discovered recently, it is theoretically possible to realize about 4 times of the luminescence efficiency compared with the known element using phosphorescence luminescence. Therefore, there have been investigated throughout the world, to begin with, the material itself, the layer composition of the emitting element and the electrode.
For example, many compounds were prepared for synthetic study by S. Lamansky et al, in J. Am. Chem. Soc, vol. 123, p. 4304 (2001) focusing on heavy metal complexes, such as an iridium complex system.
Thus, although it is a method having very high potential, the organic EL element using phosphorescence luminescence has problems to be solved. It differs greatly from the organic electroluminescence element using fluorescence luminescence in the following point. The method of controlling the location of a luminescence center, especially, to perform recombination inside the light emitting layer and to emit light stably, has been an important technical investigational work from the viewpoint of improving efficiency and lifetime of the element.
Then, there is well known in recent years the element of the multilayer lamination type equipped with the hole transport layer (located in the anode side of the light emitting layer) and the electron transport layer (located in the cathode side of the light emitting layer), both transport layers each respectively adjoining the light emitting layer (for example, refer to Patent document 1).
Especially when blue phosphorescence luminescence is used, since the blue phosphorescence emitting material itself has high T1, development of peripheral materials and control of the precise luminescence center are required strongly.
In recent years, in the light emitting layer of the organic EL element using a phosphorescence emitting material, there are disclosed as follows: the technology using a dibenzothiophene derivative as a host material (for example, refer to Patent document 1); and the technology using a dibenzothiophene derivative and a dibenzofuran derivative as a hole injection ingredient and/or a luminescence ingredient (for example, refer to Patent document 2).
However, from the viewpoint of providing an organic EL element exhibiting high light emitting efficiency with low driving voltage and excellent in heat stability and raw stock stability with long lifetime, it is still insufficient and a further solution has been investigated.
On the other hand, from the requirements of larger area, lower cost and higher productivity, the expectation for a wet method (it is called a wet process etc.) is great. Since film formation can be achieved at low temperature compared with film formation with a vacuum process, the damage of an underlaying organic layer can be reduced and it attracts great expectation from the field of improvement of light emitting efficiency and lifetime of an element.
However, in the organic EL element using blue phosphorescence luminescence, in order to realize wet film forming, it has been found that the electron transport material has a problem. From a practical viewpoint, the conventionally known electron transport materials are still insufficient in respect of solution stability and driving voltage. Further technological improvement is indispensable.